When Phoebe ends up going back to a past life, we find that she was an evil witch with very strong powers and that she, Prue and Piper were cousins rather than sisters. Phoebe is wealthy and many men from 1924 find her extremely attractive but most of her actions are merely for her own gain, something which goes completely against the ethos of The Charmed Ones.
We soon find out that an evil warlock is mixed up in the whole thing and see that it is imperative that Phoebe gets back to the present day as soon as possible before her past destroys her future.
I won't spoil it by going into any more detail, but I will say that it's a terrific episode that everyone should watch. I think you'll be glad you did!